Fuel-efficiency myths unpacked - and busted!

As the cost of fuel fuel takes an ever-bigger bite out of everybody’s budget, old wives tales about how to get the most mileage out of your fuel are resurfacing.

Some are complete nonsense, says Motor Industry Workshop Association director Vishal Premlall, while others may have held a grain of truth in the bad old days of carburettors and mechanical ignition, but simply don’t apply to modern cars with computer-controlled fuel-injection and engine management systems.

He unpacked - and shot down - some of the more commonly heard fuel-saving myths:

Shifting into neutral at stops

The moment you take your foot off the accelerator of a modern car with computerised fuel-injection, it switches automatically to ‘idle’ mode, with the injectors supplying just enough fuel to keep the engine ticking over - so there are no savings to be had there.

While there is something to be said for being able to release the clutch and unload the release bearing of a manual gearbox, on any car with a dual-clutch or automatic transmission all you’re achieving by constantly popping in and out of gear, is extra wear on the shift mechanism.

Cruising downhill in neutral

This, says Premlall, is one of the most misleading old wives’ tales - because whether you’re coasting down a hill or sitting in traffic, as long as your right foot is off the loud pedal, the car is in ‘idle’ mode and using exactly the same, minutely calibrated amount of fuel.

On a car that has energy regeneration it can actually cost you a teaspoonful or two of petrol because you’re throwing away the savings brought about by using gravity, rather than engine power, to drive the alternator.

More importantly, if you roll down a hill or up to a red light with the transmission in neutral and you suddenly need to avoid an unexpected road hazard, not only will your car’s handling under sharp steering inputs be adversely affected, you won’t be able to just put foot and get out of harm’s way.

A dirty air-filter causes higher fuel-consumption

This was actually true in the days when the engine sucked air through the filter into the carburettor; a clogged element meant it sucked harder and pulled more fuel out of the carburettor.

Today’s fuel-injection management systems suck the air over a mass airflow sensor that measures exactly how much air the engine is getting and releases precisely the right amount of petrol for optimum fuel-efficiency

But that’s not an excuse for neglecting the air-filter’s service intervals, warns Premlall; while it won’t affect fuel-efficiency, a dirty air filter can cause sluggish acceleration.

Filling up when it’s cooler saves you money

This old story is all the more pernicious because there’s a grain of truth in it. Very cold petrol is indeed marginally denser, and you get more hydrocarbon molecules - and therefore more energy - per litre. But the petrol at your local garage is pumped up from a tank so far underground (for safety reasons) that it is almost totally insulated from ambient temperature changes, so the effect is lost.

A full tank is more fuel-efficient

This one is just plain nonsense, says Premlall. Reducing the airspace above the fuel level will not reduce evaporation - because there isn’t any. Modern fuel-injection systems have a vapour recovery system rather than an open-to-air breather and some even pressurise the fuel-tank to even out the fuel pressure in the system. So any petrol that does evaporate is trapped in the tank and recovered.

Fuel-saving devices

There are lots of these devices on the market, ranging from magic magnets that are supposed to ‘align’ the molecules in the fuel by an electromagnetic field, to swirl or vortex devices that are fitted to the inlet manifold to smooth out the airflow.

There is no scientific evidence that any of them work, warns Premlall - and besides, car companies are under enormous pressure from both governments and customers to reduce the fuel consumption of their cars - don’t you think that if any of these devices actually worked, they would fit them at the factory?